1. Field of the Invention
Embodiments of the present invention relate generally to the field of electronic devices and more specifically to a technique for reducing common mode noise in differential signal interconnect.
2. Description of the Related Art
High performance electronic devices employ differential signals to transfer data at very high speeds through printed wiring board (“PWB”) interconnect between integrated circuit devices. However, one side effect of these high speed transfers is that electro-magnetic interference (“EMI”) may be created as a result of distortions of the high speed signals. This EMI may interfere with other nearby electronic devices. In order to reduce such interference, the government strictly regulates the amount of EMI that may be legally emitted from electronic devices, forcing manufacturers to limit the amount of EMI emitted from their products.
Signals in an ideal differential pair are designed to be of the same magnitude and out of the phase. Thus, the electromagnetic field generated by each interconnect within the ideal differential pair is canceled, and EMI is minimized. However, in a real word, common-mode signals are inevitably present because of imbalances in a differential pair and because of external noises injected into a differential pair, etc. EMI may be increased substantially from differential signal PWB interconnects when common-mode signals are present. Since these common-mode signals do not contribute to data signals transmitted between integrated circuit devices, filtering the common-mode signals without distorting the differential signals for the data is a desirable technique for reducing EMI.
Existing electronic devices transferring data through single-ended signal PWB interconnects employ filters to reduce noise from the data signals. One form of such filtering employs a resonator constructed from one or more of the PWB interconnects. By placing the resonator adjacent to the PWB interconnect, the resonator is able to filter the unwanted frequencies being transmitted within the PWB interconnect. Since techniques for designing a PWB resonator to filter specific frequencies is well known to those skilled in the art, designing PWB resonators to filter these additional frequencies from a PWB interconnect is not discussed herein.
One drawback of this approach is that filtering additional frequencies from both signals in a differential signal pair by using resonators located on opposite sides of the PWB interconnects that carry the differential signals may distort one signal relative to the other signal. This distortion arises from small differences in the filtering characteristics of each resonator and may lead to substantial voltage differences between the signals. Since differential signaling communicates data through small voltage differences between the signals, any resonator-induced distortion of one signal relative to the other may distort the data carried by those signals. Thus, such filtering may increase the error rate in that differential signal pair.
As the foregoing illustrates, what is needed in the art is a way to filter unwanted common-mode signals in a differential signal pair that avoids one or more of the disadvantages set forth above.